The invention relates to a belt retractor for a seat belt comprising a belt reel onto which webbing can be wound while being spring-loaded and comprising a tensioning device adapted to act on the belt reel in a winding direction. The invention moreover relates to a method of tensioning a non-used vehicle seat belt.
Belt refractors in vehicles wind up the not required webbing on a belt reel by spring actuation. On the one hand, this ensures that the non-used seat belt is safely stored. On the other hand, it is guaranteed that the fastened seat belt always contacts the vehicle occupant so that, when the seat belt locks, the vehicle occupant can be quickly decelerated.
Depending on the seating position and the body dimensions of the vehicle occupant, even in the fastened state of the seat belt part of the webbing is wound on the belt reel of the belt retractor. If in a case of restraint the belt reel is locked, it cannot continue turning for unwinding the webbing. However, since the webbing is loosely wound on the belt reel by repeated winding and unwinding, the windings of the webbing wound onto the belt reel may tighten more closely on the belt reel, thus causing small webbing length to be extended despite effectuated locking of the belt reel. This effect is referred to as film reel effect. However, this additional webbing extension influences the restraining effect of the seat belt, as the vehicle occupant is belatedly involved in the overall deceleration of the vehicle. Although tighter winding of the webbing on the belt reel could be obtained by a stronger winding spring, the webbing extension would be impeded due to the constantly acting spring force, however, and the webbing would be in uncomfortably tight contact with the vehicle occupant.